1. Field of the Invention
The present invention relates to a semiconductor device having a bipolar transistor and a method for manufacturing the same.
2. Description of the Related Art
Conventionally, there has been developed a bipolar transistor with such a construction that a base lead-out layer is electrically and directly connected to a substrate portion that is to be a collector.
In recent years, responding to demands for further miniaturization and performance enhancement of bipolar transistors, a method has been devised to form a bipolar transistor by a non-selective epitaxial growth method, using SiGe and/or SiGeC as the material for a base.
Also, as a manufacturing method of SiGe-HBT (Heterojunction Bipolar Transistor), such a method is generally adopted that an SiGe epitaxial film is grown on an Si substrate by a non-selective epitaxial method, and then polycrystalline SiGe is grown on an insulation film.
Prior arts are disclosed in Japanese Patent Laid-open No. 5-62991, Japanese Patent Laid-open No. 10-125691 and Japanese Patent Laid-open No. 11-126781.
However, when a non-selective epitaxial growth method is used to form a base of a bipolar transistor, its connection with an emitter becomes unstable, which can cause such a serious problem that the bipolar transistor does not function as a transistor. In such case, reliability of the transistor decreases.
Further, in an SiGe-HBT, SiGe on an insulating film is used as a lead-out wire to a base electrode, but its resistance cannot be lowered by an ordinary growth condition due to its insufficient film thickness. Accordingly, on a portion where the base electrode is to be formed, the resistance should be lowered by additionally forming Si film or the like on SiGe, which results in an increase of the manufacturing steps.